Generally, there are two types of ballasts employed in today's fluorescent lamps; programmed start ballasts and instant start electronic ballasts. Typically, a program start electronic ballast provides a relatively low voltage across the lamp with a separate cathode heating current during lamp startup. Pre-heating the cathode before lamp ignition lowers the amount of voltage needed to strike the lamp, that is, the glow discharge current is minimized. By minimizing the glow discharge current, the cathode life is extended since the amount of the cathode that is spattering off during lamp startup is minimized, extending the overall life of the lamp.
This type of lighting system finds particularly useful application in a setting where the lights are frequently turned on and off, such as in a conference room, a lavatory, or other setting that sees frequent but non-continuous usage. In these settings, light is needed when the room is in use, but typically the lights are turned off to save energy when no one is using the room. In short, the program start electronic ballast is beneficial for applications in which the lamps undergo a high number of on/off cycles.
Despite its advantages, the program start electronic ballast does have drawbacks. First, because it has to pre-heat the cathode before it strikes the lamp, there is a noticeable delay from the time when the light switch is activated to the time when the lamp emits visible light. Typically this delay is on the order of 1.5 seconds. This delay can be referred to as preheat or waiting time and is a disadvantage in settings where a user expects an almost instantaneous lighting of an area.
Another drawback of the program start lamp ballasts is that they are commonly utilized in a series lamp configuration. In a series configuration, if one lamp fails, it will shut down the circuit for the whole ballast, causing all lamps in the ballast to be turned off. Thus, the lamps in the ballast produce no light where they could be producing light from other lamps if the lamps were in a parallel configuration. Since all lamps will not be producing light, more frequent servicing of the lighting installation will be required, increasing the cost of labor to maintain the system.
Yet another disadvantage to the program start ballast is when lamp current is low (e.g., below 160 mA), external cathode heating can be required in order to maintain the rated lamp's life. Conventional program start ballast configurations do not address this issue of optimizing cathode heating for both preheat and operation which can minimize start time and provide energy savings.
The second conventional type of ballast, the instant start ballast, addresses some issues of the program start ballast, however, it introduces some new issues of its own. Typically, an instant start ballast does not pre-heat the cathodes, rather it applies the operating voltage directly to the lamp. In this design, at the moment the switch is turned on, a high voltage is provided across the lamp. For a typical system, the voltage can be about 600 V, and the peak voltage can be up to about 1000 V. With this high voltage across the lamp, sufficient glow current exists to bring the lamp up to a point where the lamp will ignite quickly. The lamp, therefore, has a much shorter ignition time (typically about 0.1 seconds) as compared to the program start systems, and light is seen substantially concurrently with the activation of the light switch. Also, there is no extra current drain to the cathodes during operation, since the operating voltage is applied directly to the lamp cathodes. Instant start ballasts also use parallel lamp configurations with inherently built-in redundancy in the event of the lamp failure.
However, the instant start ballast produces a glow discharge current, which degrades the integrity of the cathodes during the brief period before the lamp strikes. Over time, with instant start ballasts, the cathodes degrade at a rate, leading to an early failure of the lamp. Thus, a drawback of the instant start ballast is premature lamp failure. Because an instant start ballast burns through cathodes so quickly, lamps may fail long before their expected lifetimes.
While the program start ballasts are inefficient because they waste power, the instant start ballasts are inefficient because they may require more lamps for a given amount of time. Consequently, it is desirable to take the advantages of the beneficial aspects of the program start ballast (e.g., longer lamp life) and combine them with the advantages of the instant start ballast (e.g., quick start time) to produce an improved lamp ballast. The present application contemplates a method and apparatus that combines the positive aspects of the program start and instant start ballasts without propagating the negative aspects of those ballasts.